This invention relates to an exposure method, an exposure apparatus and a device manufacturing method. More particularly, the invention concerns an exposure method and apparatus for transferring a very fine circuit pattern onto a photosensitive substrate through multiple exposures. The exposure method and apparatus of the present invention are suitably usable for the manufacture of various devices such as semiconductor chips (e.g., ICs or LSIs), display devices (e.g., liquid crystal panels), detecting devices (e.g., magnetic heads), or image pickup devices (e.g., CCDs), or for the production of patterns to be used in micro-mechanics.
The manufacture of microdevices such as ICs, LSIs or liquid crystal panels, for example, use a projection exposure method and a projection exposure apparatus wherein a circuit pattern formed on a photomask or reticle (hereinafter, xe2x80x9cmaskxe2x80x9d) is projected through a projection optical system onto a photosensitive substrate such as a silicon wafer or a glass plate (hereinafter, xe2x80x9cwaferxe2x80x9d) which is coated with a photoresist, for example, by which the circuit pattern is transferred (photoprinted) to the wafer.
In order to meet enlargement of integration of a device (chip), miniaturization of a pattern to be transferred to a wafer, that is, improvements in resolution, as well as enlargement in area of each chip have been desired. Thus, in a projection exposure method and projection exposure apparatus which play a main role in the wafer microprocessing procedure, many attempts have been made to improve the resolution and to enlarge the exposure area in order that an image of a size (linewidth) of 0.5 micron or less can be formed in a wider range.
FIG. 22 is a schematic view of a conventional projection exposure apparatus, wherein denoted at 191 is an excimer laser which is a deep ultraviolet light exposure light source. Denoted at 192 is an illumination optical system, and denoted at 193 is illumination light. Denoted at 194 is a mask, and denoted at 195 is object side exposure light emitted from the mask 194 and entering an optical system 196 which is a reduction projection optical system. Denoted at 197 is image side exposure light emitted from the optical system 196 and impinging on a substrate 198 which is a photosensitive substrate (wafer). Denoted at 199 is a substrate stage for holding the photosensitive substrate.
Laser light emitted from the excimer laser 191 is directed by a guiding optical system to the illumination optical system 192, by which the laser light is adjusted to provide the illumination light 193 having a predetermined light intensity distribution, a predetermined orientation distribution, and a predetermined opening angle (numerical aperture NA), for example. The illumination light 193 then illuminates the mask 194.
The mask 194 has formed thereon a pattern of a size corresponding to the size of a fine pattern to be formed on the wafer 198 but as being multiplied by an inverse of the projection magnification of the projection optical system 196 (namely, 2xc3x97, 4xc3x97 or 5xc3x97, for example). The pattern is made of chromium, for example, and it is formed on a quartz substrate. The illumination light 193 is transmissively diffracted by the fine pattern of the mask 194, whereby the object side exposure light 195 is provided. The projection optical system 196 serves to convert the object side exposure light 195 to the image side exposure light 197 with which the fine pattern of the mask 194 can be imaged upon the wafer 198 at the projection magnification and with a sufficiently small aberration. As shown in a bottom enlarged view portion of FIG. 22, the image side exposure light 197 is converged on the wafer 198 with a predetermined numerical aperture NA (=sin xcex8), whereby an image of the fine pattern is formed on the wafer 198. The substrate stage 199 is movable stepwise along the image plane of the projection optical system to change the wafer 198 position relative to the projection optical system 196, such that fine patterns are formed sequentially on different regions on the wafer 198 (e.g., shot regions each covering one or more chips).
However, with projection exposure apparatuses currently used prevalently and having an excimer laser as a light source, it is still difficult to produce a pattern image of 0.15 micron or less.
As regards the resolution of the projection optical system 196, there is a limitation due to a xe2x80x9ctrade offxe2x80x9d between the depth of focus and the optical resolution attributable to the exposure wavelength (used for the exposure process). The resolution R of a pattern to be resolved and the depth of focus DOF of a projection exposure apparatus can be expressed by Rayleigh""s equation, such as equations (1) and (2) below.
R=k1(xcex/NA)xe2x80x83xe2x80x83(1)
DOF=k2(xcex/NA2)xe2x80x83xe2x80x83(2)
where xcex is the exposure wavelength, NA is the image side numerical aperture which represents the brightness of the projection optical system 196, and k1 and k2 are constants which are determined by the development process characteristics, for example, and which are normally about 5-0.7. From equations (1) and (2), it is seen that, while enhancement of resolution, that is, making the resolution R smaller, may be accomplished by enlarging the numerical aperture NA (NA enlarging), since in a practical exposure process the depth of focus DOF of the projection optical system 196 cannot be shortened beyond a certain value, increasing the numerical aperture NA over a large extent is not attainable, and also that, for enhancement of resolution, narrowing the exposure wavelength xcex (band-narrowing) is anyway necessary.
However, such band-narrowing encounters a critical problem. That is, there will be no glass material available for lenses of the projection optical system 196. In most glass materials, the transmission factor is close to zero, with respect to the deep ultraviolet region. Although there is fused silica, which is a glass material produced for use in an exposure apparatus (exposure wavelength of about 248 nm) in accordance with a special method, even the transmission factor of fused silica largely decreases with respect to the exposure wavelength not longer than 193 nm. It is very difficult to develop a practical glass material for a region of an exposure wavelength of 150 nm or shorter, corresponding to a very fine pattern of 0.15 micron or less. Further, glass materials to be used in the deep ultraviolet region should satisfy various conditions, other than the transmission factor, such as durability, uniformness of refractive index, optical distortion, easiness in processing, etc. In these situations, the availability of practical glass materials is not large.
As described, in conventional projection exposure methods and projection exposure apparatuses, the band-narrowing of the exposure wavelength to about 150 nm or shorter is required for formation of a pattern of 0.15 micron or less upon a wafer 198 whereas there is no practical glass material for such a wavelength region. It is, therefore, very difficult to produce a pattern of 0.15 micron or less on a wafer.
Recently, an exposure method and apparatus for performing a dual exposure process, comprising a periodic pattern exposure and a standard (ordinary) exposure, to a substrate (photosensitive substrate) to be exposed, has been proposed in an attempt to produce a circuit pattern including a portion of 0.15 micron or less.
Here, the term xe2x80x9cstandard exposurexe2x80x9d or xe2x80x9cordinary exposurexe2x80x9d refers to an exposure process by which an arbitrary pattern can be photoprinted although the resolution is lower than that of the periodic pattern exposure. A representative example of it is the exposure process to be performed by projection of a mask pattern with a projection optical system.
A pattern to be printed by the standard exposure (hereinafter, xe2x80x9cstandard exposure patternxe2x80x9d) may include a very fine pattern less than the resolution. The periodic pattern exposure is a process for forming a periodic pattern of a similar linewidth as that of the very fine pattern.
Such periodic pattern exposure may use a Levenson type phase shift mask, for example. An example of a dual exposure process is shown in FIGS. 1A-1C. A periodic pattern (FIG. 1A) and a standard exposure pattern (FIG. 1B) are printed on the same position, by which a very fine pattern (FIG. 1C) corresponding to a composite image of them is produced.
In this manner, a pattern to be produced finally is photoprinted as a standard exposure pattern, but, since the standard exposure pattern contains a pattern portion lower than the resolution, a predetermined portion of a periodic pattern having high resolution is printed at the same position as the pattern lower than the resolution. As a result of it, the resolution of the standard exposure pattern can be improved and, finally, a desired pattern including a very fine line smaller than the resolution can be produced.
More specifically, in order to improve the resolution of a standard exposure pattern (FIG. 1B), a high resolution periodic pattern (FIG. 1A) is printed on the same position. In such a dual exposure process, if the elongation direction of the fine-line portion of the pattern of FIG. 1B is registered with the periodicity direction in FIG. 1A, no particular problem arises.
If, however, a standard exposure pattern includes fine lines of different directions, such as shown in FIG. 2B wherein there are fine lines extending in the same direction as the periodicity and fine lines extending in a direction perpendicular thereto, while the fine lines in the same direction as the periodicity may be resolved, the fine lines extending perpendicularly to the periodicity may not be resolved.
Details will be described with reference to a pattern called a gate pattern or a T gate pattern, used with a positive type resist material, in conjunction with FIGS. 1A-1C and 2A-2C. It is assumed now that in these drawings the periodic pattern comprises such a pattern that light passes therethrough by which its phase is inverted. This periodic pattern has a periodicity not less than 2. The standard exposure pattern comprises such a pattern that light passes through the peripheral portion around the pattern which blocks light, and it has a binary amplitude with a constant phase.
For example, in FIGS. 1A-1C, each fine line of the gate pattern of FIG. 1B (standard exposure pattern) is oriented in the same direction as the periodic pattern of FIG. 1A. Thus, the resolution of the fine line of the gate pattern of FIG. 1A (standard exposure pattern) can be increased.
In the example of a T gate pattern shown in FIG. 2B, there are additional fine lines extending like a T-shape, orthogonally to fine lines of a gate pattern. Thus, there are fine lines extending in different directions.
If there are fine lines extending longitudinally and laterally, resolution is particularly difficult to achieve in such a zone (hard-resolution zone), i.e., pattern spacings xe2x80x9cAxe2x80x9d, where a fine line and a pattern are juxtaposed with each other with a spacing not larger than the resolution. In order to attain improved resolution for such a zone, use of a periodic pattern such as shown in FIG. 2A is necessary. However, the mere use of such a periodic pattern would result in disconnection of each fine line extending in a direction perpendicular to the periodicity, although resolution may be accomplished for the hard-resolution zone.
Therefore, when a dual exposure process using a periodic pattern and a standard exposure pattern is to be performed, a pattern to be produced finally is limited in some cases, depending on the orientation of the periodic pattern used. Particularly, as regards a pattern having fine lines extending in a direction different from the periodicity direction of the periodic pattern, it is difficult to well meet the same, with the dual exposure process used conventionally.
It is accordingly an object of the present invention to provide an exposure method, an exposure apparatus and/or a device manufacturing method, by which, when a multiple exposure process is to be performed by using plural mask patterns being different in image contrast, every fine line (including a case of a complicated pattern having fine lines extending in different directions) can be reproduced successfully.
In accordance with an aspect of the present invention, there is provided an exposure method for printing, upon a photosensitive material, a pattern having fine lines of an odd number extending about a certain point, through a multiple exposure process, characterized in that a phase shift mask having an even number of boundaries defined with a phase difference of 180 deg. between adjacent regions about the point, is used, wherein the number of the boundaries is larger than the number of fine lines.
In accordance with another aspect of the present invention, there is provided a multiple exposure method using plural masks, characterized in that a position where an image of a low-contrast pattern is to be formed on the basis of a mask, of the masks, having fine lines extending in different directions, is exposed with an image of a pattern of a mask having a phase difference of 180 deg. between adjacent patterns, whereby a contrast of an exposure amount distribution related to the pattern of the low contrast is increased.
In accordance with a further aspect of the present invention, there is provided an exposure apparatus having an exposure mode for performing an exposure method as recited above.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.